Two days ago I raised a question about why so much of western US was showing up with a warm temperature anomaly when the Precipitable Water map reading was so low relative to most other places. Yesterday and today show the same thing happening, all of which has called for a need to dig in and do some research. Every time there are signs of extra heating there must be a physical cause of some sort and we need to find it. It will either be an input of more heat-producing energy effects, compared to what is normal for the day, or an absence of the normal cooling effects produced by things like low cloud cover. The problem is always aggravated, and will be today, by having poor information about what “normal” really means for any given checkpoint under consideration.
I think I have learned something from this particular situation that is helpful, and want to share it with anyone who is just interested or may be thinking about doing this same kind of work. The western US is an area beset by fires and smoke-filled air these days, which crossed my mind but doesn’t seem promising enough to pursue. It is also an area of high elevation, a lead more worth following. I thought to open up a regular Weather Map that I seldom have need for, called “Sea Ice/Snow Cover,” because it has an additional feature that could prove useful. It shows the relative elevation of land all over the globe with rudimentary color coding that is just right for the purpose of a quick reference. Here it is:
Outside of the Himalayas the US west stands out with its broad expanse of really high elevation, in company with several other regions that are very close to the same. Almost all of the large inland places of this type you see on the map either contain real deserts or experience unusually dry climate conditions for a good part of the year. Out of curiosity I also checked them all out on the animated PWat website to see if their high elevation made any difference in the movement of water vapor streams. For this one 5-day period at least it seems that all high-altitude streams of water vapor for some reason preferred to mostly skirt around these regions instead of passing over. Try it. I think this will be something worth watching for in similar situations in the future. For now it opens up the possibility that only a small amount of extra overhead water vapor that may not even be part of a visible stream could be enough to raise the surface air temperature by several degrees above normal. The leverage principle that is so effective in the polar regions works the same way—although to a lesser degree—for any region that has a relatively low average PWat reading to begin with.
While uncertain, I believe it is possible that the regular historical PWat reading for the elevated US west at this time of year could be as low as 7-8kg. Meanwhile the recorded reading for today on the Weather Map averages out at around 9kg. Assuming these numbers are right, we have a 25-30% increase in total atmospheric vapor content today, which is normally enough to produce an anomaly of +3-4C anywhere, and is what we see recorded for the US west on this map:
Carl